Electrodeposition of metals on ferrous and nonferrous substrates is a well known method for providing corrosion protection and for providing improved cosmetic appearance. Heretofore, such electrodeposition has been carried out by essentially two types of electroplating baths.
One such electroplating bath is cyanide-based. However, the use of cyanide electrolytes present significant ecological problems and require expensive waste treatment equipment. Moreover, cyanide baths are toxic and tend to embrittle certain sheets and exhibit low current efficiencies.
In an effort to overcome the deficiencies of the cyanide baths, chloride-based baths of three types were developed. The three types of baths were termed neutral, ammonia-based and non-ammonia-based. Although the chloride-based baths eliminate the toxicity problem of the cyanide baths, these baths also have other limitations. The neutral and ammonia-based baths contain excessive amounts of ammonium ions and/or chelates, thereby making metal removal costly and difficult. The non-ammonia-based baths generally result in deposits which are brittle at thicknesses over 0.5 mils and which tend to flake at thicknesses less than 0.5 mils. Furthermore, iron co-deposition, which causes a dull appearance and poor corrosion protection, is a problem with all chloride-based baths.
Examples of such chloride-based zinc baths are discussed in detail in U.S. Pat. Nos. 3,694,330; Re 27,999; 3,729,394; 3,730,855; 3,778,359; 3,787,297; 3,838,026; 3,855,085; 3,928,149; 4,070,256; 4,075,066; 4,089,755 and 4,119,502 (all incorporated herein by reference). The acid zinc and neutral zinc plating baths disclosed in these patents relate to electroplating of zinc, rather than a zinc-nickel alloy, and the deposited zinc plate is necessarily thin and has poor corrosion resistance unless it is protected by appropriate post treatment. The established products in the plating industry is to post treat all zinc electroplate to provide a chromate conversion coating, such as described in U.S. Pat. Nos. 3,694,330; 3,919,056 and 4,119,502. The pure zinc electroplate has good receptivity to conventional chromate conversion coatings which greatly enhance the corrosion resistance. Best corrosion protection is obtained when using thicker highly colored coatings or iridescent chromate coatings, but the more transparent, lighter colored coatings, such as the blue chromates, enhance the luster and are preferred for decorative eye appeal in spite of the inferior corrosion protection.
Heretofore the electroplating industry was not aware of any practical economical process for substantially increasing the corrosion resistance of chromated bright zinc plate, and, therefore, the utility of articles provided with such plate was necessarily limited, although the protection could be improved by use of lacquer coatings and the like.